Filamentary cathode



July s, ms

- A. K. WING, JR

FILAMENTARY CATHODES Filed March 18, 1942 Patented July 6, 1943 UNITEDSTATES PATENT OFFICE FILAMENTARY CATHODE Arthur K. Wing, .lr., Chatham,N. J., assignor to Radio Corporation of America, a corporation ofDelaware 5 Claims.

My invention relates to cathodes for electron discharge devices,particularly to filamentary cathodes for power and transmitting types ofsuch devices.

Self-supporting cathodes for power tubes have been proposed comprisingstraight parallel filaments arranged in a cylindrical surface with thefilaments attached at one end to support and current supply conductors.Experience has shown that the long lengths of untensioned filament wiremay either bow and buckle at operating temperature, or, if heldstraight, expand unequally and force the cathode off center. Electrodespacers of insulating material to brace the cathode are not feasible inmany high powered tubes, because of the severe voltage and temperatureconditions. It is undesirable to stiffen the filaments by using thickerwire because of the increased heating current, and because of thereduced effective cathode emitting area of the large diameter filaments,for a given cathode power.

An object of my invention is a self-supporting filamentary cathode forpower tubes which has a large effective emitting surface, which may bemade with relatively long spans of straight filaments that will not bowor buckle, or move off center, and which is rugged in construction andeasy to manufacture.

The characteristic features of my invention are defined in the appendedclaims and one preferred embodiment thereof is described in the folowing specification and shown in' the accompanying drawing in whichFigure 1 is an elevational view, partly in section, of a power tubeembodying my improved filamentary cathode, Figures 2 and 3 l aredetailed views of my cathode taken along sectional lines 2-2 and 3-3,respectively, of Figure 1, Figure 4 is a detailed View of one of thefilament conductors of my improved cathode, and Figure 5 is a detailedview taken along sectional line 55 of Figure 1.

For convenience of illustration my improved cathode is shown in a powertube with a conventional fluid cooled anode I enclosing and concentricwith a grid electrode 2, and my improved cathode 3. The cathodecomprises a plurality of parallel straight filaments 4, of high meltingelectron emissive metal such as tungsten or thoriated tungsten, arrangedin a cylindrical surface. The support for the cathode comprises a seriesof metal blocks 5 arrangedin a circle at the lower end of the cathode,each block being carried on the end of a heavy support rod and currentsupply conductor 6, which in turn may be supported on conventionallead-in conductors, not shown.

The opposite end of the cathod stands free and, when constructedaccording to my invention, requires no brace or interelectrode spacer.The filaments are joined together at their upper ends to make a rigidunitary cage structure that will not move off center during operation,and yet the filaments are sufficiently free to expand Without" bowing orbuckling. Each filament at its upper end is bent through an obtuse angletoward the center of the cathode, where all the filaments are boundtogether. Preferably, a short end section ib of each filament isstraightened into a line parallel with the center line of the cathode,and against the side of a round metal plug 1. The diameter of the plugmay be so chosen with respect to the wire diameter and number of wiresthat the end sections 41) li in side-by-side contact around the plug. Aring 8 may be pressed over the wires and plug to hold the end sectionsin place, and if desired, the plug, ring and filament and sections maybe fused by arc welding.

To prevent relative movement between the filaments, as when thefilaments may swing or rotate about their junction at the plug, a metalwedge or spacer is conveniently placed between each radial portion 4a ofthe filament and intermediate the operative cylindrical surface andcenter of the cathode. A cap 9, preferably comprising a disc or coneintegral with the ring, may be provided with a downwardly extendingflange IE, the flange being provided with a series of evenly spacednotches into which each filament is guided as the cap is fitted over theends of the cathode. The cantilever anchorage of each filament effectedby fixing the radial portion 4a of each filament wire at two spacedpoints, effectively prevents lateral movement of the upper ends of thefilament wires.

The filament wires are preferably arranged in pairs so that filamentheating current flows in one direction in two adjacent filaments or pairand in the opposite direction through the next pair. By operating tworound wires in parallel, greater ffective emitting surface is providedthan if the two wires were consolidated in a single round conductor. Thelarger number of fine filaments produces a more uniform electronemission from the cylindrical surface of the cathode and affords ahigher tube perveance. The accurately bored holes are made in each metalblock 5, the holes being just large enough to snugly receive the ends ofthe filament wires 4. A slight taper in the tungsten .040 inch indiameter.

current supply conductors 6 grouped and contwo holes may, if desired, beprovided to tightly wedge the filaments in an upstanding positionparallel to the center line of the tube. If desired the lower ends ofthe filaments may be passed a short distance through the blocks and. theends riveted or welded.

Since current flows in the same direction through the two wires of anyone pair, and in the opposite direction through the adjacent pair, thereis a tendency, caused by electromagnetic fields produced by the filamentheating current, for the wires of the pair to move together,

According to another characteristic feature of my invention, the wiresare braced at one or more points to prevent bowing. The tie-wire llpasses between the two filament wires of one pair and is looped one ormore times around each filament. An offset bend in the filamentelfectively prevents the looped tie-wire from slipping lengthwise of thefilament. The ends of the tie-wire may conveniently extend to anotherpair of filaments having the same voltage and polarity where the wireloops about this second pair and hence on to a third or fourth pair,,ifdesired. In the particular cathode illustrated, twenty-four filamentsare shown, the

twelve pairs of filaments being tied together in groups of six so thatfour tie-wires engage all of the filaments.

. Good results have been obtained in making a cathode about 5.25 incheslong, from the blocks to the plug 1, and 1.75 inches in diameter withtwenty-four round filament wires of thoriated With alternate nected forsingle phase operation, the heating current for the filaments is about420 amperes at 8.5 volts for heating the filaments to their normaloperating temperature of about 2050 K.

In manufacture, the filaments are pre-formed with the bends at 40. and4b and with the offset bends for the tie-wires. The tie-wires oftungsten with thier loops are likewise pre-formed and the filaments andtie-Wires threaded together, whereupon the lower ends of the filamentsare inserted in the bored holes of the metal blocks 5 and welded inplace. The metal plug 7 of tungsten is then held in place while the endsections 41) of the filaments are grouped around the plug and while thecap 9 of molybdenum and 1.1 inches in diameter is pressed over theassembly with the notches of the cap flange guided over the filaments.Finally the plug, ring and filament end section are fused together by anelectric arc. The cathode assembly is flashed and carburized and is thenmount ed with the grid and other electrodes of the tube. The finishedtube is preferably operated with the filaments hanging, that is with theplug 1 below the conductors 6.

My improved cathode is self-supporting, has a large effective emittingsurface, and comprises long spans of straight filaments that will notbow or buckle. My improved cathode, even without interelectrode spacersor a center support rod, is a rigid unitary structure that will not moveoff center during operation. My improved cathode is rugged inconstruction and easy to manufacture.

I claim:

1. A cathode comprising a plurality of parallel straight filamentsarranged in a cylindrical surface, the end of each filament at one endof the cathode being bent radially inward and bound together in a singlerigid mechanical junction on the axis of the cathode, metal spacersbetween adjacent wires intermediate said surface and said junction, theopposite ends of the filaments being connected to current supplyconductors, and tie-wires intermediate the ends of the cathode betweenpoints on the filaments of like voltage and polarity.

2. A cathode comprising a plurality of parallel straight filaments in acylindrical surface, the filaments being connected to and supported bycurrent supply conductors at one end, and being bent radially inward attheir other end and joined together at the axis of the cathode, andmeans to fixedly space the filaments at their said other ends comprisingmetal spacers between the filaments intermediate said surface and saidaxis.

3. A cathode comprising a pluralityof circularly arranged parallelstraight filaments, bends in the ends of the filaments, with said endsjoined together at the center of the filament circle, a metal cap withuniformly spaced notches in its rim, said cap being secured over thejoined ends of the filaments with each filament passing through one ofsaidnotches.

4. A cathode comprising a plurality of cylindrically arranged uniformlyspaced parallel filaments, the filaments being joined together at oneend, the opposite ends of the filaments being connected to currentsupply conductors, two adjacent filaments being connected to oneconductor and the next two filaments being connected to anotherconductor.

5. A cathode comprising a plurality of spaced parallel filamentsarranged in a cylindrical surface, a plurality of current supplyconductors at one end of the filaments, the filaments being arranged inside-by-side pairs, the two adjacent filaments of each pair beingconnected to one of said conductors, a short tie-wire to brace apart thefilaments of each pair, each tie-wire being joined to each filament ofits pair intermediate the ends of the filament.

ARTHUR K. WING, JR.

